The present invention relates generally to demodulators for digital radio communications systems and more specifically to a phase controlled demodulator of the type wherein a frequency sweeping operation is effected when there is a large frequency difference between a received carrier and a recovered carrier.
In a satellite communications system, the demodulator at a receiving end suffers carrier frequency fluctuations due to the instabilities of the local oscillator of the satellite transponder. Frequency sweeping technique is currently employed to search for a missing carrier when the frequency deviation exceeds the phase control range of the demodulator. When the demodulator is pulled into the phase control range, the sweeping operation must be terminated to allow normal closed loop operation to be resumed. However, the conventional phase lock detector takes long to detect when the demodulator is phase locked and requires complex external circuitry.